There is a lot to cover with regards to optimising lighting and build times:
First I would recommend you look at your lightmass settings. There are a lot of settings you can tweak to significantly improve your build times and it is covered in detail in the Unreal Engine documentation I have linked below. At the end of the documentation there’s a long section on getting the best lighting build times
Documentation:
The next best thing to do is add a Lightmass Importance Volume if you don’t have one already. You probably have many meshes at the edge of your world that do not need high quality lighting. Lightmass will emit photons for those meshes, so the larger your levels the higher your build times unless you use a Lightmass Importance Volume. The Lightmass Importance Volume controls the area that Lightmass emits photons by lowering the lighting quality outside the volume and improving your build times. You can play around with the volume until you have the best build performance without compromising your visual aesthetics.
Documentation:
I would also recommend you watch the Lighting with Unreal Engine Masterclass:
Or here is a summary of the Masterclass:
Another good video on lighting optimisation with a focus on light mobility: